a. Field of the Invention
The invention relates to track retarders. More particularly, the invention relates to brake shoes associated with track retarders and the like.
B. Description of Prior Art
Railcar or track retarding mechanisms are used to slow down or stop the movement of cars as they are switched from track to track in the course of classifying or assembling trains in freight yards. Retarders are generally in the nature of mechanically positioned, spring-loaded opposed friction surfaces which rub against the vertical wheel surfaces of railcars as they pass between the gripping surfaces. Friction thus generated between the wheel and the gripping surfaces acts to remove kinetic energy from the railcar, which in turn will reduce its speed of movement.
Often a loud squealing or shrieking noise is generated when the friction surfaces are pressed against the rotating wheels; the noise thus produced is most often in the frequency range of 2200 to 3400 Hertz with an amplitude (or level) which may rise to 130dB(A) at distances of 50 feet or more.
The retarders are equipped with replaceable brake shoes which are brought to bear against the sides of each wheel (although sometimes only the wheels on one side of the railcar will be so affected). The braking pressure may vary from a few hundred pounds to several tons, depending upon the amount of energy which is to be removed from the railcar. Most often the brake shoes are of rolled or cast steel to possess sufficient strength to withstand the braking action.
Positioning of the brake shoes may be accomplished by a motor-driven gearbox or lever arrangement, hydraulic rams or pneumatic cylinders. Contact between the moving wheels and the stationary brake shoe results in a "stick-slip" condition which in many cases causes harmonic vibrations in the plate or center disc area of the railcar wheels. It is this vibration which produces the objectionable screeching sound previously described. Most of this stick-slip phenomena occurs between the inside of the wheels and the inside brake shoes. (Inside refers to those brake shoes between the rails as opposed to those brake shoes which are positioned outside of the rails.)
The prior art has been directed toward special braking materials to suppress noise levels from track retarder operation. U.S. Pat. No. 3,321,048 describes a ductile iron as a braking material. U.S. Pat. No. 3,716,114 describes a non-metallic noise suppressant brake material. Material selection, however, does not per se address all parameters involved in noise generation from track retarder operation. A brake shoe that addresses additional parameters beyond noise suppression should contribute to improved track retarder operation from an environmental, operational and safety standpoint.